1. Field of the Invention
The present invention broadly relates to electroconductive inks and electroconductive coatings containing carbon fibrils. More specifically, the present invention relates to screen printable inks or coatings that contain oxidized or nonoxidized carbon fibrils.
2. Background
Polymers which contain or have mixed therein an electrically conductive additive or filler are generally referred to in the art as electroconductive composites. These composites are often formed in an effort to obtain a compound combining desired attributes found in selected polymers (e.g., flexibility, durability, etc.) with those found in the selected fillers (e.g., conductivity, etc.)
One type of electroconductive composite is electroconductive coatings, which are thin electroconductive composites applied, directly or electrostatically, onto substrates such as automotive body parts. Known electroconductive coatings are comprised primarily of a polymeric binder which contain or have mixed therein a lesser amount of electroconductive filler such as finely divided particles of metal such as silver, gold, copper, nickel, palladium or platinum and/or carbonaceous materials like carbon black or graphite. The polymeric binder may attach the conductive filler to the substrate and/or hold the electroconductive filler in a conductive pattern which serves as a conductive circuit. In practice, the two key parameters for measuring an electroconductive coating are its conductivity and thickness. Thus, different amounts of polymeric binder and electroconductive filler are used to achieve different levels of conductivity and thickness.
In practice, a convenient method of creating an electroconductive coating is by using an electroconductive ink. In one embodiment, the electroconductive ink is an electroconductive coating in liquid form (i.e., where the polymeric binder is a liquid at room temperature and the electroconductive filler is dispersed therein). Such electroconductive inks are described in U.S. Pat. No. 5,098,771 to Friend entitled “Conductive Coatings And Inks,” hereby incorporated by reference. Friend describes a composite suitable for application to a surface comprising polymeric binder and less than 30% carbon nanotubes, preferably less than 15% and even more preferably between 0.5 and 10 percent. (All percentages by weight based on nanotubes plus binder.) The coatings made by the conductive inks of Friend have bulk resistivity between 10 exp −2 and 10 exp 6 ohm cm, and preferably between 10 exp −1 and 10 exp 4 ohm cm. In another embodiment, the electroconductive ink contains three components: a polymeric binder, an electroconductive filler and a liquid vehicle. The liquid vehicle includes solvents (e.g., liquids which dissolve the solid components) as well as non-solvents (e.g., liquids which do not dissolve the solid components). The liquid vehicle serves as a carrier to help apply or deposit the polymeric binder and electroconductive filler onto certain substrates.
Once applied to a substrate, the electroconductive ink is dried (e.g., the solvent or liquid vehicle is vaporized or evaporated), and an electroconductive coating is formed from the remaining polymeric binder and electroconductive filler.
Unlike electroconductive coating, however, a key parameter for measuring electroconductive inks is viscosity. In particular, the viscosity of the electroconductive ink should be such that the ink will not “run” (i.e., spread horizontally in an undesirable fashion) or “bleed” (i.e., spread vertically in an undesirable fashion) when applied onto the substrate, otherwise the resulting electroconductive coating will not form with the proper or desired thickness, conductivity or at the proper location. Electroconductive inks which use a liquid vehicle are known to present various running and bleeding problems.
Furthermore, depending on the use of the electroconductive ink, thixotropy may also another important parameter for measuring electroconductive inks. Unlike viscosity, which measures the ability of the liquid to withstand shear force, thixotropy measures the ability of the liquid to change its viscosity in response to a shear force. Complicated applications may require electroconductive inks which are both viscous and thixotropic. Certain sophisticated applications such as screen printing further require that the thixotropy property be such that the viscosity of the ink will decrease in response to a shear force so that the ink can be forced through a screen. Thus, certain uses, such as screen printing will require an ink with a different set of rheological properties (i.e., viscosity, thixotropy) than others such as spray or ink jet applications where only viscosity may be important.
For this reason, current electroconductive coatings and electroconductive inks contain significantly greater amount of polymeric binders than electroconductive fillers. It was believed that the polymeric binder acted like a glue and thus was essential in electroconductive coatings to keep the electroconductive fillers in place or to attach them to the substrate, as well as in electroconductive inks to prevent the ink from running or bleeding. Thus, commercial carbon inks and coatings typically contain a greater weight percentage of polymeric binders than the electroconductive filler. The larger presence of polymeric binder, however, limits the overall conductivity of the electroconductive ink or coating.
The inventors have discovered, however, that the amount of polymeric binder needed in electroconductive coatings and inks can be eliminated or significantly reduced when using carbon fibrils of the present invention as an electroconductive filler. As a result, the inventors have also discovered that conductivity of the electroconductive coating or ink can be significantly improved.